HOW THE BRAIN MAKES POTENTIAL 181showed how such responses in the cortex tuned responses of other neu-
rons and altered neuron connections. For example, reception of specifi c
structure in the cortex feeds back to “enforce statistical constraints,” as
the researchers put it, on incoming stimuli. Th ese constraints help further
extract the higher- order informational structure, eliminate alternative
possibilities, and resolve ambiguity during perceptual and cognitive
inference.^12
In another study, Jarmo Hurri and Aapo Hyvärinen analyzed videos
of natu ral scenes by breaking them down into frames (or small patches
of the total frame) with tiny time delays of 40–960 milliseconds. Th ey
produced evidence that responses in cells, even to simple features like
lines, are dependent on higher- order spatiotemporal correlations. As they
went on to say, a more generative model is needed in which hidden vari-
ables can be used to interpret the under lying real world.^13
Using a similar approach, Selim Onat and co- researchers showed that
the continuity of the movie stimulus, rather than a mixture of bits from
diff er ent movies, was crucial for forming a coherent image “in which the
inherent spatio- temporal structure of natu ral movies were preserved.”
Th e results suggest “that natu ral sensory input triggers cooperative
mechanisms that are imprinted into... primary visual cortex.”^14
All this is consistent with what has been revealed about the anatomy
of brain connections. Communication is not one way, from the outside
in, or the bottom up. Th ere are abundant reciprocal or top- down connec-
tions from cortex to sensory waystations in the brain. For example, the
ret i nae send signals to the lateral geniculate nuclei in the midbrain, which
send them on to the visual cortex. But, as with other sensory waystations,
the lateral geniculate nuclei receive far more signals from the cortex than
they send to it.
More generally, studies of rat, cat, and monkey brains show that the
average input/output connection ratio between diff er ent regions of the
cortex is close to one. Th is suggests the strong role of reentrant (feedback)
mechanisms, in which pro cessing of inputs to cortical centers are rapidly
used as feedback to fi rm up information from the sensory inputs.
Th is densely recurrent wiring organ ization has sometimes puzzled in-
vestigators. Susan Blackmore, for example, referred to the “currently mys-
terious profusion of descending fi bres in the visual system.”^15 But that is
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